TERMS AND CHARACTERISTICS OF HOMOGENEOUS IGNITION OF COAL-WATER PARTICLES COATED WITH A WATER FILM

The problem of coal-water fuel ignition of particles in a high-temperature heating has been solved numericallyin the framework of a two-layer model of "coal-water". The basic characteristics of the integrated ignition has been shown. The influence of the vaporization process of the water film on the ignition conditions has been shown.


Introduction
One of the most promising ways to reduce the environmental load of thermal power plants (TPP) (reducing emissions of nitrogen oxides and sulfur) coal power plants, both in Russia [1] and abroad [2] is the transition from the traditional ways of organizing combustion (e.g., flaring [3] or layers [4]) to the burning of coal in the various fuel compositions, for example -coal-water fuel (CWF).Despite a long history of development of this technology (the beginning of work on the WCF dates back to the year 1959 [5]), it has not been widespread in the modern thermal power plants (with the possible exception of China and Japan).Likely, it is a consequence of poor drafting of basic aspects of the theory of ignition and burning of complex way designed (multicomponent) and essentially inhomogeneous (on the fractional composition and physical state of the components) fuels.
Designed to date model ignition [2,[6][7][8] drops of WCF are based on the hypothesis of a uniform distribution in the amount of the basic elements of the system "waterorganic and mineral part of fuel".However, it has been established experimentally [9] in the process of spraying the fuel in the combustion chamber may be formed of water-coal particles, which are "pillars" [9] (homogeneous particle) carbon coated with a thin film of water.The likelihood of such a configuration is quite high (P ≥ 50%) [9], due to the nature modes of training systems and fuel atomization.It is obvious that the mechanism of ignition particles WCF will differ significantly from the ignition of solid [10] (dry coal, wood) and liquid (oil, diesel fuel) [11] fuels.It's safe to say that a drop of WCF will not ignite until complete evaporation of the surface of the film.Accordingly, there is a high probability of formation of inhomogeneous (temperature) torch [to the presence of "cold zones" (most likely in the initial section of flow)].
As a result, there are risks of violation of temperature combustion.As a consequence of possible open flame.[12] In this case it is necessary to stabilize the temperature field of the torch through the fuel [13] (gas, fuel oil) or electromagnetic [14] (microwave, plasma) "illumination".The latter is very difficult and leads to an increase in expenses for reconstruction and operation of the boiler equipment.
The purpose of our workmathematical modeling of ignition of coal-water fuel particles in a corresponding firing installations boiler as part of a two-layer structure of the "coalwater" and the definition of the main characteristics and conditions of the ignition of such a particle WCF.

Physical and Mathematical Statement
At the initial time (t = 0), a drop of coal-water fuel is in a corresponding combustion chamber of the boiler unit.Under the influence of convection and radiation heat flux near surface film of water begins to evaporate.As a result, the particle size decreases.Evaporation rate (and hence the thermal effect of the phase transition) depend on the temperature of the surface water layer.The process of thermal decomposition and ignition is initiated after removing of the film particles WCF in a continuing heating process.It is assumed that particles on the surface of carbon oxidation reaction occurs: The mathematical formulation of the problem, the corresponding stated higher of physical model is a non-stationary system of differential equations in partial derivatives: There is 4 the boundary condition of the fourth kind at the interface of the "homogeneous coal -water film": The system of equations (1-3) has been solved with the following boundary conditions and closing ratios: The rate of chemical reactions has been calculated from the mathematical expression of the Arrhenius law: The designations: Тare the initial temperature of the particles, K; Т сis the temperature of the environment, K; Т ௪ିis the temperature at boundary section of the "coalwater", K; r ଵcoal particle radius, m; rexternal WCF particle radius, m; εis a constant radiation of a black body; αcoefficient of convective heat transfer, , Where: ‫܂‬is the temperature at the boundary of evaporation, K; ‫܂‬is the temperature of the liquid corresponding to the freezing point, K; ‫܅‬the velocity of evaporation at a temperature, ‫ܕ(‬ • ‫)ܛ‬ ⁄ ; ૄmolar Mass of water vapor, ⁄ ; Runiversal gas constant, ( • ) ⁄ .The system of nonlinear equations ( 1) -( 9) has been solved by finite difference method [15], in two stages.The first problem of evaporation [on the heating surface applied boundary condition (6)] in the second ignition [on the outer boundary condition (7)] has been solved.
Figure 1 shows the dependence on the delay time of ignition homogeneous water coal particles coated with a water film, the temperature of the environment.The particle size has been varied within a fairly wide (but at the same time, most likely for the modern nozzle technology [9]) the range of δ = 0,1 ÷ 0,5 • 10 -3 m.Dependency analysis (Fig. 1) shows that the temperature of the external oxidant has a significant impact on the dynamics of the processes of preparation and thermal ignition particulate of WCF.So we can say that for a particle diameter of δ = 0,5 • 10 -3 m temperature increase at 500K (from 900 K to 1400 K), significantly accelerates ignition [approximately 5 -6 times (delay time decreases from 16 seconds to 3 )].The latter can indicate the important role of the complex processes of evaporation of the water film, the warm of base layer of fuel and the thermal decomposition of the organic carbon in the temporal characteristics of the thermal treatment steps.At the same time it should be noted that in case of fire drops WCF diameter δ = 0,1 • 10 -3 m, the degree of this influence is negligible, which may also indicate the important role of the temperature distribution along the radius of the particle.Consequently, the application of balance-type models [16] during the predictive modeling of ignition of fuel can lead to large errors in determining the ignition delay times.Figure 2 shows the dependence of t i (particles of coal-water fuel diameter δ = 0,1 • 10 -3 m) on the ambient temperature by varying the thickness of the water film (Δ = 0,05 ÷ 0,15 • 10 -3 m).The analysis of the relationship shows that the film of water affects the dynamics of the thermal preparation and ignition, but the extent of this impact is negligible.Thus, when the ambient temperature T e = 1100K threefold increase in the thickness of the water layer (with 0.05 to 0.15 • 10 -3 m) leads to a slight increase in the induction period.Likely, it indicates that the base layer of heating fuel occurs when the water film is upon evaporation.Accordingly, the following initiation of combustion of the coal particles (already "dehydrated") proceeds considerably faster.

Conclusion
According to the results of numerical modeling the basic temporal characteristics of processes of heat and ignition homogeneous preparation of coal-water fuel particles coated with a water film have been set up.It has been shown that an increase in the thickness of the water film has a negligible (0.1 to 2%) impact on the delay time of ignition.The latter, probably due to the fact that the evaporation of the water film occurs heating of the base layer of fuel, resulting in subsequent ignition "dehydrated" particles is faster than the same particles of the dry coal.
The research has been carried out at the expense of the grant of the Russian Science Foundation (project № 15-19-10003).
x energy equation for a homogeneous coal: a Corresponding author: ssyrodoy@yandex.ruDOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 201 chemical kinetics for homogeneous coal:(2)energy equation for the film of water: